Fix overriding MGOS_DEBUG_UART on ESP8266

CL: Fix overriding MGOS_DEBUG_UART on ESP8266

Author: rojer

platforms/esp8266/Makefile.build

@@ -28,7 +28,7 @@ BUILD_LIB_NAME ?= lib.a
 APP_SOURCE_DIRS = $(sort $(dir $(APP_SOURCES)))
 APP_FS_DIRS = $(sort $(dir $(APP_FS_FILES)))
 
-MGOS_DEBUG_UART = 0
+MGOS_DEBUG_UART ?= 0
 
 APP_PATH := $(CURDIR)